Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique in which a user transmits on many orthogonal frequencies (or subcarriers). The orthogonal subcarriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system allows some subcarriers to be assigned to different users, rather than to a single user. Today, OFDM and OFDMA technology are used in both wireline transmission systems, such as asymmetric digital subscriber line (ADSL), and wireless transmission systems, such as IEEE-802.11a/g (i.e., WiFi), IEEE-802.16 (e.g., WiMAX), digital audio broadcast (DAB), and digital video broadcast (DVB). This technology is also used for wireless digital audio and video broadcasting.
In conventional OFDM networks, a fixed data rate may be used and the amount of transmission power may be adjusted based on how far away the user is from the transmission node. Alternatively, data may always be transmitted at full power, but if the user is close to the transmission node, a higher order modulation and a weaker code are used. On the other hand, if the user is far away from the transmission node, a lower order modulation and a stronger code are used. For some OFDM networks, two antennas may be used to transmit two data streams to one user, resulting in a doubled data rate. However, at a cell edge, the two data streams may interfere with each other to such an extent that the modulation and coding have to be backed off too far for successful communication. Therefore, there is a need in the art for an improved method of communicating with users in a MIMO system.